1. Field of the Invention
This invention relates to a charge storage device and to a method of operating such a device.
2. Description of the Related Art
FIG. 1 of the accompanying drawings shows an example of a pixel 101 of a known charge storage device in the form of an image sensor 100. Generally, the image sensor 100 will comprise a two-dimensional matrix of N rows and M columns M of pixels with associated row 102 and column 103 conductors. However, in the interests of convenience, only one pixel is illustrated in FIG. 1.
The pixel 101 comprises a photosensitive diode PD and a switching diode SD coupled in series between the associated row conductor 102 and associated column conductor 103. In the example shown, the switching diode SD and photosensitive diode PD are arranged with the cathodes coupled together. A capacitor C is shown coupled across the photosensitive diode D. This capacitor C may be the parasitic capacitance of the photosensitive diode PD or may be an additional capacitor added to increase the dynamic range of the image sensor 100.
Each column conductor 103 is coupled to a suitable charge sensitive amplifier 104 having a capacitive coupling between its input and its output.
Such image sensors suffer from image lag resulting from the non-linear forward resistance of the switching diode SD which causes the resistance of the switching diode SD to increase as the voltage across the diode drops. Thus, light falling on the photosensitive diode PD of the pixel 101 causes the capacitance C of the photosensitive diode PD to be discharged. When an appropriate voltage V.sub.R is applied to the associated row conductor 102 to forward bias the switching diode SD, a current starts to flow to recharge the photosensitive diode PD capacitance C enabling the charge stored at the photosensitive diode PD to be integrated by the charge sensitive amplifier 104. However, as the voltage across the photosensitive diode PD rises, the voltage across the switching diode SD decreases and so the forward resistance of the switching diode SD increases. The rate of charging of the photosensitive diode capacitance C thus slows down and at the end of the readout period, the photosensitive diode capacitance C will not have been completely recharged. The row voltage V.sub.R falls at the end of the readout pulse and the switching diode SD again becomes reverse-biased. Even when light does not fall on the photosensitive diode PD in the integration period between readout pulses, when the next readout pulse is applied to the row conductor 102, the switching diode SD is again forward-biased because charging of the photosensitive diode capacitance C was not completed during the previous readout pulse. A small amount of current thus flows and the photosensitive diode capacitance C is charged a little more. This process repeats for the next few readout pulses with, each time, the amount of charging becoming a little smaller.
FIG. 2a illustrates graphically the change in the row voltage V.sub.R with time and shows the application of four sequential readout pulses R.sub.0, R.sub.1, R.sub.2 and R.sub.3 at times t.sub.0, t.sub.1, t.sub.2 and t.sub.3, respectively for the situation where no light has fallen on the pixel since just before the first readout pulse. FIG. 2b illustrates the change in the voltage V.sub.X across the photosensitive diode capacitance C for the time period within which the four row voltage pulses are sequentially applied. As can be clearly seen from FIG. 2b, although no light has fallen on the photosensitive diode PD since before the first row voltage pulse R.sub.O, the capacitance of the photosensitive diode PD is not fully recharged during the first readout pulse R.sub.O and is recharged a little more during the following readout pulses R.sub.1, R.sub.2 and R.sub.3. The current that flows is integrated by the charge sensitive amplifier 104. FIG. 2c illustrates the change in voltage of the output of the charge sensitive amplifier 104 over the time scale of the four readout pulses R.sub.0 to R.sub.3 with the times t.sub.0, t.sub.1, t.sub.2 and t.sub.3 representing the commencement of the application of the corresponding readout pulses R.sub.0, R.sub.1, R.sub.2 and R.sub.3. Instead of the output voltage V.sub.O dropping from a high voltage V.sub.H to a low voltage V.sub.L at the end of the first readout pulse R.sub.O as indicated by the dashed line a in FIG. 2c, the voltage V.sub.O has, as indicated by the solid line b, a much slower transition towards the low voltage V.sub.L, because, as explained above, the photosensitive diode capacitance C continues to charge during readout pulses following the first readout pulse R.sub.O after the illumination has been removed. Accordingly, there is a "lag" in the sensed image which means that any moving or changing image may appear blurred.
EP-A-233489 describes a charge storage device comprising an array of storage elements for storing charge, the storage elements being arranged in rows and columns with the storage elements in a column being coupled to a first conductor and the storage elements in a row being coupled to a second and to a third conductor, each storage element in a row being coupled to the associated second conductor by a first rectifying or isolation element and to the associated third conductor by a second rectifying or isolation element with the first and second rectifying elements allowing the passage of current when forward-biassed by applied voltages.
As described in EP-A-233489, the storage elements comprise photosensitive diodes while the first and second rectifying elements comprise switching diodes. In operation of the device, appropriate voltages are applied to the respective second and third conductors associated with a row of storage elements to reverse-bias the associated first and second rectifying elements when charge is to be stored at the charge storage elements of a row and to forward-bias the associated first and second rectifying elements when charge is to be read from the charge storage elements of a row.
EP-A-233489 thus replaces the switching diode or rectifying element SD with two rectifying elements coupled in series between two row conductors and couples the photosensitive diode PD to a junction between the two rectifying elements. The pixel is then read out by applying voltages to the row conductors that will forward-bias both of the rectifying elements or switching diodes so that a current flows through the two switching diodes defining a voltage at the junction which, if the two diodes are identical, will be equal to the average of the voltages applied to the two row conductors. In operation of such a charge storage device, when a pixel has just been read out and the capacitance of the photosensitive diode has been charged, the switching diodes are reverse-biased by applying appropriate voltages to the two row conductors. As light falls on the photosensitive diode PD, charge will be produced and the voltage across the photosensitive diode PD will fall. When the next readout pulse is applied to the two row conductors to forward-bias the switching diode, current will flow through the photosensitive diode capacitance C until the average of the voltages of the two row conductors is reached. Thus, the photosensitive diode capacitance C can be completely recharged within the corresponding readout pulse and there is accordingly little or no lag.
However, the charge storage device illustrated in EP-A-233489 requires two row conductors for each row of charge storage elements which necessarily increases the amount of area within the device which must be taken up by the conductors and, moreover, requires a greater number of connections between conductors and corresponding driving circuits for applying voltages to the row and column conductors.
The increased number of row conductors inevitably increases the area occupied by conductors within the device at the expense of the area available to the storage elements. This may be of particular disadvantage where the charge storage device is an image sensor and the image sensor needs to be as transparent as possible to enable, for example, a display such as a CRT (Cathode Ray Tube) or LCD (Liquid Crystal Device) display to be viewed through the image sensor or where it is desirable to have as large a photosensitive area as possible, for example where the device is an image sensor for use in X-ray diagnostics.